When supporting pipe strings in well bores, the vertical load is usually carried by surface gripping arrangements situated on elevators or spiders of drilling or servicing rigs. When circumstance requires that surfaces be gripped and forces be applied that tend to slip the gripping means along the surface, the gripping means needs teeth or the equivalent. Pipe gripping dies, especially, are the result of needing hard materials used with high unit loads to impress toothed profiles into pipe surfaces to keep dies, in pipe string handling apparatus, from slipping on pipe. Most pipe string lifting apparatus have die carrying slips that apply light loads against the pipe surface until vertical pipe loads are experienced, then the dies engage the pipe with forces that are proportional to the load being lifted. The teeth milled into the gripping face of the die are usually forced to some small amount into the surface being gripped. The teeth are subject to damage. Providing the needed teeth on a removable insert that can be easily installed in and removed from the slips reduces the expendable cost. Slips are inherent in elevators and spiders as the wedge shaped elements that convert vertical loads to radial loads on pipe gripping dies. In addition to wedge shaped slips there are several forms of equivalent means to apply radial forces for gripping pipe. The term slip will be used herein to embrace such equivalent features.
Pipe gripping die inserts are usually dove tailed, or T shaped and, during installation, are pushed along the dove tailed, or T shaped, grooves in the slip to be fitted. The grooves usually hold four or more die inserts in stacked, serial, fashion. The usual elevator or spider may have three or more stacks of die inserts distributed about the periphery of the pipe accepting bore of the pipe supporting system. When pipe loads are lifted, the upper die inserts add their supported load to the inserts below. Accumulation of vertical forces along the grooves tend to press the lowest die insert into the surface used to axially restrain each stack of die inserts in the related groove.
Surfaces are rarely perfect, and dies are rarely situated in perfect cylindrical geometry when prepared to grip pipe. Some dies carry more than their proportion of the load. When lifting pipe strings within the load rating of the elevator, some die inserts experience loads that may be a multiple of their statistically proportional load. Such die inserts may crush through the axial restraints of the confining groove. In addition to slip damage, the die inserts could fall into the well being worked, and pipe strings themselves can be dropped.
When specific die inserts experience serious overload, the size data stamped on their back, or smooth side, are often clearly impressed into the mating groove in the slip. The dies are much harder than the slips. Such image transfers are not smeared, indicating that the die inserts did not slip axially in their groove after axial loads were experienced.
The die insert slippage problem is not new. U.S. Pat. No. 6,264,395 issued Jul. 24, 2001 addresses the problem by providing intermediate barrier rings so that a fewer number of die inserts are stacked serially between barriers to reduce the ultimate force on each axial barrier. The needed result is achieved, at substantially increased construction cost. The present invention distinguishes in that there is no reduction in the number of die inserts stacked in each series. Instead, the radially loaded surface on the die insert, contacting the slip, is given more reluctance to slide.
During manufacture and maintenance procedures, die inserts are skidded along their related slip groove during installation and the grooves are designed and built with the ease of that operation in mind. When overloaded die inserts print their stamped numbers in the related groove there is no indication that they lose any ability to move axially along their related slip groove when die inserts are being changed out for servicing. The die inserts are not ground after stamping and the stamping action upsets a small amount of metal, which is subsequently hardened. The upset metal represents a very small surface area until it is driven into the surface of the mating groove. Acceptance of the impression into the groove material redistributes the total load and unit loading is reduced, avoiding damage beyond acceptable marring.
It is possible and desirable to alter the usually smooth surfaces on die inserts, that contact slip groove surfaces, to increase the acceptable load carrying power of elevators and spiders. Otherwise stated, it is desirable to reduce the overload damage to details of elevators and spiders, when operated within their overall load rating.
These and other objects, advantages, and features of this invention will be apparent to those skilled in the art from a consideration of this specification, including the attached claims and appended drawings.